This error usually happens when the user running Source Integrity Standard (SIS) doesn't have write access to the project working file, which is the *.pj file in the project directory, or the project archive, which is often in the rcs subdirectory.
If you aren't having any problems with checking in and out members, then the problem is likely with the project archive. Can you check that the non-admin user can create a new file in the archive directory? There will be a read-only flag on the archive file, but the user should have all other rights to the archive directory and files.
If the user can create a new project and checkpoint that project, then you could compare the file and folder permissions on the new and old projects.